Shape memory materials feature an ability to transform shape from a temporary, frozen, shape to a permanent shape when triggered by an environmental stimulus, such as heat, light, or vapor. Used creatively, this phenomena can be exploited for a wide range of applications. Many polymers intrinsically show shape memory effects, e.g., on the basis of rubber elasticity, combined with vitrification or crystallization but with varying characteristics such as strain recovery rate, work capability during recovery, and retracted state stability. Among the first shape memory polymers (SMPs) reported as such was crosslinked polyethylene, which was discovered and patented in 1971 by Radiation Appliances, Inc., and a methacrylic acid ester reported by the Vernon-Benshoff Co. for use as a denture material. The mechanism of strain recovery for such a material was identified as being far different from that of shape memory alloys (SMAs), which are based largely on nickel-titanium alloys.
More particularly, a shape memory polymer is a super-elastic rubber, when the polymer is heated to a rubbery state, it can be deformed under resistance of ˜1 Mpa modulus, and when the temperature is decreased below either a crystallization temperature or a glass transition temperature, the deformed shape is fixed by the lower temperature rigidity while, at the same time, the mechanical energy expended on the material during deformation is stored. Thereafter, when the temperature is raised above the transition temperature (Tm or Tg), the polymer will recover to its original form as driven by the restoration of network chain conformational entropy. The properties of the SMPs will be closely linked to the network architecture and to the sharpness of the transition separating the rigid and rubber states. Compared with SMAs, SMPs have an advantage of (i) high strain, to several hundred percent because of the large rubbery compliance while the maximum strain of a SMA is less than 8%. (ii) facile tuning of transition temperatures through variation of the polymer chemistry; and (iii) processing ease at low cost.
A concurrently filed application for patent filed by inventors herein discloses the synthesis and characterization of thermally stimulated SMPs having different thermomechanical properties and their use in various applications, including as medical devices and mechanical actuators. The disclosed materials span a range of room temperature moduli, from rigid glassy materials having storage moduli of several GPa to compliant rubbers with moduli as low as tens of MPa. Moreover, the retracting (rubbery) moduli have been tuned over the range 0.5<E<10 MPa, as dictated by the end application. One example of such an SMP is chemically crosslinked polycyclooctene (PCO), a stiff semicrystalline rubber that is elastically deformed above Tm to a temporary shape that is fixed by crystallization. Fast and complete recovery of gross deformations are achieved by immersion in hot water. Other SMPs offering tunable critical temperatures and rubber modulus have been synthesized using a thermosetting random copolymer formed from two vinyl monomers that yield controlled Tg and casting-type processing. Such copolymers were crosslinked with a difunctional vinyl monomer as crosslinker, the concentration of crosslinker controlling the rubber modulus and thus the work potential during recovery. In addition to their shape memory effects, these materials are also castable allowing for processing more complex shapes and they are optically transparent. The use of chemical crosslinking in both of these cases, however, limits the types of processing possible and permanently sets the equilibrium shape at the point of network formation.
Semicrystalline thermoplastic polymers with sharp Tg>room temperature and low crystallinity are also good candidates for shape memory, while offering the advantage of melt processing above Tm, allowing repeated resetting of the equilibrium shape by relaxing stress in the fluid state. Representative of the polymers in this class of SMPs are polyurethanes whose soft domains are semicrystalline with low melting points (but higher than Tm) and whose hard domains feature a higher melting point only exceeded during processing. The effect of the hard segments and soft segments on the shape memory effects have been investigated. In addition to such polyurethanes, block copolymers of polyethylene terephthalate (PET) have been synthesized for their shape memory effects.
Miscible blends of a semicrystalline polymer with an amorphous polymer have been investigated in the past, though not with respect to shape memory behavior, due to their attractive crystalline properties and mechanical properties. For those blends that are miscible at the molecular level, a single glass transition results, without broadening, an aspect important to shape memory. Additionally, in such miscible blends the equilibrium crystallinity (which controls the plateau modulus between Tg and Tm where shape fixing is performed) also changes dramatically and systematically with blend composition; i.e., relative levels of each component. Although numerous blends of this type have been investigated, there has been no known disclosure of the utilization of such blends for their shape memory properties.